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<h1><a href="https://archiveofourown.org/works/27573454">irresistible</a> by <a class='authorlink' href='https://archiveofourown.org/users/kinkykawa/pseuds/kinkykawa'>kinkykawa</a></h1>

<table class="full">

<tr><td><b>Series:</b></td><td>youngblood (miyacest one-shots) [7]</td></tr>

<tr><td><b>Category:</b></td><td>Haikyuu!!</td></tr>

<tr><td><b>Genre:</b></td><td>Age Difference, Aged Down Miya Atsumu, Alpha Miya Osamu, Alpha/Beta/Omega Dynamics, Alternate Universe - Canon Divergence, Barebacking, Breeding, Breeding Kink, Creampie, Dubious Consent, Implied Mpreg, Incest, M/M, Mating Bites, Mating Cycles/In Heat, Omega Miya Atsumu, Omega Verse, PIV Sex, Sibling Incest, afab language, he is not de-aged he is just magically handwaves younger brother now, to resolve any confusion</td></tr>

<tr><td><b>Language:</b></td><td>English</td></tr>

<tr><td><b>Status:</b></td><td>Completed</td></tr>

<tr><td><b>Published:</b></td><td>2020-11-15</td></tr>

<tr><td><b>Updated:</b></td><td>2020-11-15</td></tr>

<tr><td><b>Packaged:</b></td><td>2021-05-06 21:01:36</td></tr>

<tr><td><b>Rating:</b></td><td>Explicit</td></tr>

<tr><td><b>Warnings:</b></td><td>Creator Chose Not To Use Archive Warnings, Underage</td></tr>

<tr><td><b>Chapters:</b></td><td>1</td></tr>

<tr><td><b>Words:</b></td><td>2,069</td></tr>

<tr><td><b>Publisher:</b></td><td>archiveofourown.org</td></tr>

<tr><td><b>Story URL:</b></td><td>https://archiveofourown.org/works/27573454</td></tr>

<tr><td><b>Author URL:</b></td><td>https://archiveofourown.org/users/kinkykawa/pseuds/kinkykawa</td></tr>

<tr><td><b>Summary:</b></td><td><div class="userstuff">
              <p>Atsumu's on top of him, straddling his hips, hands pressed to his chest as he whines, breathless, “‘Samu-nii.”</p><p>His breathing is a little short. His scent is everywhere, sweet and intoxicating. He’s moving on top of Osamu, little rolls of his hips, and fuck, the front of Osamu’s sweatpants is already damp, is that <i>slick—</i></p><p>“You’re in heat,” he says, slightly stunned, because — Atsumu’s in <i>heat.</i></p><p>(Atsumu lives to cause problems, which is why he presents as an omega while alone with his older brother.)</p>
            </div></td></tr>

<tr><td><b>Relationships:</b></td><td>Miya Atsumu/Miya Osamu</td></tr>

<tr><td><b>Series:</b></td><td>youngblood (miyacest one-shots) [7]</td></tr>

<tr><td><b>Series URL:</b></td><td>https://archiveofourown.org/series/1933474</td></tr>

<tr><td><b>Comments:</b></td><td>18</td></tr>

<tr><td><b>Kudos:</b></td><td>531</td></tr>

</table>

<a name="section0001"><h2>irresistible</h2></a>
<div class="story"><div class="fff_chapter_notes fff_head_notes"><b>Author's Note:</b><blockquote class="userstuff">
      <p>written as a comm for <a href="https://twitter.com/osamyaatsum">ceyri</a> who wanted to go full problematic and stuff this with kinks, including: de-aged atsumu, omegaverse, age gap, breeding kink, atsumu in heat and osamu fucking him through it. and i was too happy to oblige, because 1) we really need more de-aged atsumu in the world, and 2) osamu deserves to fuck his baby brother and knock him up.</p><p>osamu is about 21-ish (college) in this fic, atsumu is probably 16 (entering high school). people present with pup heats/ruts when they're around 12-14 but don't experience full heats much later. osamu's been eye-fucking his brother since he presented, then fucked off to tokyo before he could do anything, but lol if he thought he was safe. atsumu's heat gets triggered by osamu and his scent, and well 8)</p><p>mind the tags before reading! <b>atsumu's physiology gets described with neutral/afab language</b>, but i opted not to tag this as "trans male character" because he's not necessarily trans? if that makes sense?? if you guys feel it's better to tag it, tho, let me know and i will! author is queer. haven't really edited this so if i spot anything i'll just fix it later.</p><p>HAVE FUN let us all sin together and enjoy it.</p>
    </blockquote></div><div class="userstuff module">
    
    <p>When their parents said Atsumu wanted to come visit him in Tokyo, Osamu’s first instinct had been to say no.</p><p>He has several reasons, all of them perfectly valid. Doesn’t want his teenage brother hanging around when he finally has a break between terms. Doesn’t want Atsumu in his space, messing around with his things, spoiled teenager that he is. Could do with some peace and quiet, some time to himself, laze around without a little brother to look after.</p><p>But then Atsumu, fucking <em> brat</em>, whines <em> ‘Samu-nii, please? </em>And, well.</p><p>This is another reason: Osamu is entirely too weak to his younger brother.</p><p>Hard to control himself, sometimes, when he smells chocolate — practically tastes it at the back of his tongue. Hard to keep his hands to himself when Atsumu’s nearby, beside him, on his bed, on the couch, half in his lap. Fantasies filled with the scent of cocoa and of omega, lingering on the clothes Atsumu borrows (steals). Mischief in honeygold eyes and a toothy grin.</p><p>And now that sweet-cocoa scent is here, in his tiny apartment, seeping into his floors as Atsumu lies sprawled on the tatami. He’s playing some gacha game on his phone, Osamu doesn’t know. He’s doing his best to ignore Atsumu as he prepares them dinner (tuna onigiri, at Atsumu’s request, it was a long train ride and he’s <em> hungry</em>).</p><p>“Thanks, <em> nii-san</em>,” Atsumu drawls, taunting, when Osamu finally drops a plate in front of him. Osamu just rolls his eyes and takes a seat on the other side of the small, low table.</p><p>“Shut up and eat,” he mutters, reaching for his laptop.</p><p>He settles on continuing his Conan binge watch, easy to play in the background, doesn’t need much paying attention. Grabs one of the rice balls and picks up a few pieces of pickled radish (Atsumu better thank him for breaking out the good stuff). Settles down to eat.</p><p>“So what d’you wanna do tomorrow?” he asks, because he honestly has no idea what to do with Atsumu. His grand plan for the two weeks of term break had been to laze around and practice cooking. Play some games. Hit the gym. Maybe get into a pick-up volleyball game or two.</p><p>Atsumu shrugs, careless. “D’know. Maybe the Skytree?”</p><p>Osamu stares at him. “What are you, a tourist?”</p><p>His brother steals a piece of his radish. Brat. “I mean, what else d’you do in Tokyo? Shibuya? Akihabara? Kabuki-cho?”</p><p>“I am <em> not </em>taking you to Kabuki-cho.”</p><p>“Boring.” Atsumu makes a face. Osamu pinches his fingers with his chopsticks. “<em>Hey.</em>”</p><p>“Fuck, just — fine, I’ll take you to the Skytree. We can get sushi after.”</p><p>It should <em> not </em>make him feel like this, the sight of Atsumu’s sunny grin, but Osamu still feels warm in his ribs. Fucking hell, he needs to get a grip. He stuffs the rest of his onigiri into his mouth, turning to watch Conan knock Mouri out yet again to solve another case. Safe thoughts, simple thoughts, nothing at all to do with the way his brother’s licking at his own fingers, picking off the stray grains of rice and flakes of tuna.</p><p>Fucking hell, Osamu should never have agreed to this. Or should have at least stocked up on sake before Atsumu arrived.</p><p>There’s a soft, disconcerted sound from beside him. Osamu glances up, frowning. Atsumu’s pink-cheeked and frowning, something a little — off about his scent. It makes Osamu set down his food, leaning across the table to touch the back of his hand to Atsumu’s forehead. Warm.</p><p>“You okay?”</p><p>“Mmn.” Atsumu wrinkles his nose, shakes his head. “Fine. Prob’ly just tired from the trip.”</p><p>And that’s yet <em> another </em> thing. Osamu owns a tiny <em> studio </em>apartment, just a kitchen and an open space with a table, closet, cabinet, shelves, and futon. He’s got a spare futon, sure, but nowhere else to put it but right beside his. But Atsumu’s here now, and there’s nothing else Osamu can do but accommodate. And keep his hands to himself.</p><p>“Go to bed early then,” he says, pulling his hand back and returning to his dinner. “If you’re still not feeling well tomorrow morning, we can stay in.”</p><p>“Yes, <em> auntie,</em>” Atsumu snipes. Osamu pinches his fingers again. Atsumu steals the last of his radish. But if he’s got it in him to be that bratty still, munching happily on his rice and tuna, he can’t be feeling too bad. Maybe he is just tired.</p><p>Osamu leaves Atsumu to take care of the dishes while he lays out the spare futon (and yes, he <em> did </em> remember to air it out before Atsumu came, thanks very much <em> mother</em>). Hesitates a moment before unrolling it beside his own, table pushed out of the way. Shuts down his laptop and digs out his sleeping clothes, sweatpants and a tank top. Hesitates.</p><p>Here is his dilemma: does he let Atsumu go in first, then shower in a bathroom that will smell of his little brother, in water he touched? Or does he go in first then spend the next fifteen minutes willing himself <em> not </em>to get a boner while trying not to think of Atsumu in the shower?</p><p>Fortunately, Atsumu takes the decision out of his hands by grabbing his own clothes. “I call first shower,” he says over his shoulder, already halfway to the door. Osamu rolls his eyes in token protest.</p><p>His own shower is long and distracted, and ends with a blast of freezing water. Tries not to think of the lingering cocoa scent in the air. Dries off quick and reminds his dick to behave.</p><p>Steps back into the room to the sight of Atsumu tucked into the spare futon, turned towards Osamu’s own. Cheeks a little flushed, lips lightly parted as he breathes. Osamu thinks he recognizes that shirt, thieving little brother that he’s got, just a little too big on him.</p><p>Tears his eyes away to go turn off the light, then slides into his own futon, curled as far away from Atsumu as he can get. Osamu drifts off to the sweet scent of cocoa, Atsumu just a few feet away, asleep.</p><p> </p><p>He wakes up far too soon, still dark, something heavy shifting on top of him and when Osamu groans, inhaling, he tastes chocolate at the back of his tongue.</p><p>Wide awake in seconds because that’s <em> Atsumu </em>on top of him, straddling his hips, hands pressed to his chest as he whines, breathless, “‘Samu-nii.”</p><p>His breathing is a little short. His scent is <em> everywhere, </em> sweet and intoxicating. He’s moving on top of Osamu, little rolls of his hips, and fuck, the front of Osamu’s sweatpants is already damp, is that <em> slick—  </em></p><p>“You’re in heat,” he says, slightly stunned, because — Atsumu’s in <em> heat. </em>Not the pup heat when he’d presented, temperature like a mild fever, Atsumu tucked into his nest with ice cream and juice. Now he’s leaking slick onto Osamu’s clothes, voice thin as he whines, “‘Samu-nii, I can’t — I don’t—”</p><p>(Must have been due to go into heat around this time, triggered by a room full of alpha scent. Osamu making his food, <em> taking care of him, </em>of course his instincts would go haywire.)</p><p>“Wait, ‘Tsumu, get off—” Needs to get <em> away, </em> lock himself in the bathroom, lock Atsumu in the apartment and <em> leave. </em>Can’t be around him, not when he smells like this, when he looks like the subject of Osamu’s every wet dream and fantasy. Atsumu is his brother but Osamu’s self-control is hanging by a thread and if Atsumu pushes—</p><p>“Alpha,” plaintive, <em> pleading. </em> Then his impossible, terrible little brother fumbles the front of his sweatpants down — wraps his hand around Osamu’s cock — <em> rubs it against his slit—  </em></p><p>Osamu has Atsumu on his back in seconds, teeth pressed to the omega’s pulse as a warning, behave, <em> be still. </em>Slides his hands up Atsumu’s thighs, not even wearing underwear anymore, slick between his legs. This close, his cocoa scent is overwhelming, pooling in Osamu’s lungs. It’ll probably linger on this futon for days.</p><p>(God, he hopes it does.)</p><p>“Yeah, I got you,” he says, breathless, hushed. Atsumu whines again and Osamu complies, leaning in to kiss him. Atsumu’s mouth is soft, pliant, parting curiously as Osamu licks at the seam of his lips. His cunt opens easily, too, one finger then two as Osamu eases them inside tight, wet heat. Swallows the moan low in Atsumu’s throat.</p><p>It’s so simple, then, to kiss his way down Atsumu’s body — the length of his neck, the dip of his collarbone, the taut skin of his navel. Nips teasingly at the ridge of his hip, smirking when Atsumu cries out. Licks down the seam of his thigh, tasting salt-sweat, then dips his tongue into slick to taste what he’s dreamed of for what feels like forever.</p><p>When Atsumu comes, it’s with slick thick on Osamu’s tongue, clenching around three of Osamu’s fingers. Thighs locked around his older brother’s head, hands in his hair, soft whine of <em> ‘Samu-nii, please.  </em></p><p>If he asks that sweetly, how can Osamu say no?</p><p>Runs his hand through the mess between Atsumu’s legs, fingers coming away sticky-wet. It shouldn’t feel this hot, lubing his cock with his brother’s slick, but Osamu’s long past caring. He rubs the head of his cock against wet folds, is he even going to fit? Eases himself in, just the tip, then pressing in slowly, inch by inch until he’s buried inside—</p><p>“Fuck, ‘Tsumu,” he gasps, breath hot on his brother’s shoulder. It’s <em> tight, </em>so good, better than anything he’d ever imagined. Turns his head to kiss Atsumu, inhales cocoa and heat-scent. </p><p><em> Alpha, </em> his brother whimpers again, and Osamu presses his forehead to Atsumu’s, panting open-mouthed against Atsumu’s lips as he <em> moves.  </em></p><p>Tight, so tight, hot and wet around him. Osamu keeps his thrusts shallow, not slow but deliberate, working Atsumu up. He builds into languid rolls of his hips, until his cock slides in-out, dragging every time he pulls out. Revels in the scratches Atsumu leaves, nails dragging over Osamu’s back as he arches into him.</p><p>“Like that?” Osamu taunts, grinning. “Look at you. In heat because of me, couldn’t resist my scent, could you?”</p><p>“No— <em> ah!” </em>Atsumu breaks into another cry as Osamu thrusts sharply, sinking down to the hilt, fingers pressing bruises into skin. </p><p>“Such a mess,” he murmurs, trailing his mouth over the flush coloring Atsumu’s collar, “and it’s all for me.”</p><p>Can feel his knot starting to catch, and fuck, the temptation to stuff Atsumu with it is too much. Sweet little brother, almost <em> sobbing </em> for it, incoherent now except for Osamu’s name and <em> alpha </em> and <em> please </em> and <em> more. </em>Osamu fucks his knot into Atsumu, pressing in as deep as he can get, feels Atsumu clench and shudder under him. And when his brother throws his head back, baring the line of his throat—</p><p>Cocoa floods his mouth as Osamu sinks his teeth in, right over the scent gland, hard enough to break skin.</p><p>It takes a while for them both to come down; Osamu laps at the bite mark on Atsumu’s neck, shushing the omega with a kiss when he whimpers. They’re still locked together, and every shift of Osamu’s hips makes Atsumu squirm. He reaches down, traces a finger around where his cock is still buried inside his brother. Keeps his hand there when he finally pulls out, dragging fingers through the mess of cum and slick and fucking it all back into Atsumu, filthy squelching sounds loud in the tiny room.</p><p>Watches as Atsumu comes a third time, futon a wreck underneath him. His body is littered with hickeys; his hair is matted to his forehead, skin flushed and sweat-damp. He’s gorgeous. Osamu wants him so badly it aches.</p><p>He tugs Atsumu around when his brother starts blinking dazedly, sleepily. They’ll have to share his futon, but he’s got no problem with that. Atsumu curls against him, temporarily sated, but Osamu knows enough to realize he’ll be aching for it again soon. That’s fine, though; they have all week to get Atsumu through it. </p><p>(Fingertips tracing lightly over the soft give of his brother’s pelvis. Osamu’s going to fuck him so good that by the end of it, he’ll be swollen with cum.)</p><p>For now, though, he brushes his mouth over Atsumu’s hair, tucking him in close. Let him sleep; Osamu already has plans for how to wake him up in the morning.</p>
  </div><div class="fff_chapter_notes fff_foot_notes"><b>Author's Note:</b><blockquote class="userstuff"><p>CHEERS TO KINKS i'll drink to that. i'm on twitter as <a href="https://twitter.com/kinkykawa_">@kinkykawa</a> if you guys wanna say hi and chat with me about more problematic content or just read my occasional OH SHIT TWINCEST HOT tweets. i also open comms/requests periodically if y'all want more sexy, sexy twincest 👀</p></blockquote></div></div>
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